The present invention relates to a steam isolation valve system, and more particularly, to a main steam isolation valve for a boiling water reactor capable of making smooth steam flow in a valve system in the boiling water reactor.
Generally, in a boiling water reactor (BWR), as shown in FIG. 13, a reactor pressure vessel 1 located in a reactor containment vessel 4 is connected to a steam turbine 3 through a plurality of, for example, four, main steam lines 2, and the respective main steam lines 2 are provided with first and second main steam isolation valves 5 and 6, respectively, in an arrangement in which the first steam isolation valves 5 are disposed inside the reactor containment vessel 4 and the second steam isolation valves 6 are disposed outside the reactor containment vessel 4. Accordingly, the reactor pressure vessel 1 can be isolated as occasion demands by closing these main steam isolation valves 5 and 6.
The steam after working in the steam turbine 3 is then condensed in a condenser 7, and thereafter, the thus condensed water is returned to the reactor pressure vessel 1 through a water supply system 8. As shown in FIG. 13, Venturi tubes 19 are provided for the main steam lines 2, and third main steam valves 9 are also provided for the main steam lines 2. A reference numeral 10 denotes a header.
Next, with reference to FIG. 14, each of the main steam isolation valves 5 (6) has a valve body 11 provided with steam inlet side portion 11a and steam outlet side portion 11b which are connected with each of the main steam lines 2, and the valve body 11 is further provided with a valve disk accommodation portion 11c in which a valve disk 12 of a bottomed cylindrical structure is accommodated in a manner slidable in its axial direction.
The valve disk 12 has an inclination of about 45.degree., for example, with respect to the flowing direction of the steam as shown by arrows to reduce flow resistance. The main steam isolation valve 5 (6) is provided with a driving device 14 to which a drive shaft 13 is connected at its one end and the other end of the drive shaft 13 is connected to the valve disk 12 so that the valve disk opens or closes the steam flow passage by axially reciprocating the valve disk 12 by the operation of the driving device 14. The driving device 14 is mounted to the bonnet 17 by a Yoke Rod 18. The driving device 14 is composed of an air cylinder 14a, an oil cylinder 14b, outer springs 14c, a spring seat 14d and a coupling 14e.
When the valve disk 12 is lowered and abuts against a valve seat 16, the steam flow passage is fully closed and conversely, when the valve disk 12 is lifted as shown in FIG. 14, a valve port is fully opened. In the fully opened state, the valve disk 12 is positioned at a portion at which a valve bottom of the valve disk 12 is positioned to a portion to half close the inlet side portion 11a of the valve body 11 for reducing a moving stroke thereof between the fully opened position to the fully closed position of the valve disk 12.
In order to ensure the position of the valve seat 16, a line axis of the flow passage of the inlet side portion 11a is shifted from the same direction as the line axis of the main steam line 2 to a direction perpendicular to the axis of the valve disk 12 as approaching the valve seat 16. On the other hand, a line axis of the flow passage of the outlet side portion 11b, directing from the valve seat 16 to the outlet, is shifted from substantially the same direction of the axis of the valve disk 12 at the valve seat 16 to the axis direction of the main steam line 2 as approaching the outlet. Accordingly, the flow path changes its direction in three curved portions and throttled at the valve seat 16.
According to the conventional structure of the main steam isolation valve 5 (6) described above, the steam flow from the inlet side portion 11a is rapidly bent or curved at the inlet side flow passage, and at the portion between the valve disk bottom and the valve seat 16, then throttled and flows as a jet flow into the flow passage of the outlet side portion 11b, after which the steam flow is gradually enlarged. As this result, a vortex current of the steam will be caused around the jet flow at the bottom portion of the valve disk 12 and the valve seat side of a duct of the outlet side portion 12b, and a relatively large turbulence is caused in comparison with a case of a straight steam line axis including no curved portion.
According to the steam flow condition described above with respect to the conventional main steam isolation valve, pressure loss is caused due to the curving of the flow passage, the rapid throttling thereof, and the rapid enlargement thereof in the first and second main steam isolation valves 5 and 6. The pressure loss reduces the steam energy working for the turbine. Accordingly, it will be said that plant operational efficiency can be improved by reducing the pressure loss of the first and second main steam isolation valves.
Further, the steam flow turbulence causes flow-induced vibration of the valve disk 12, which may cause wear or damage to the valve disk 12, the valve shaft 13 and structural members contacting thereto such as inlet guide rib 15. Such problem or inconvenience damaging soundness of the plant structure will be likely caused to machineries or mechanisms such as valves, for example, the third main steam valves 9, disposed downstream side of the steam flow.